There doesn't seem to be any such tradition of shaving babies in Polish culture. I'm wondering where the OP got this idea from. It exists in certain Far Eastern countries and within the Muslim and Hindu cultures.
postrzyżyny to be the moment of the symbolic differentiation between a little boy and a little girl
Similar to the western European tradition of dressing a boy in skirts and keeping his hair long. The occasion of donning trousers for the first time was called 'breeching'. It died out gradually but was still very popular in Victorian England. Lots of photos of boys with lovely dresses and beautiful long curls!
no official ceremony for girls
Well there wouldn't be as girls kept their hair long. The nearest thing would be the putting up of the hair when they reached marriageable age. In Poland they had some ceremony involving hair before the wedding when the hairstyle was changed from two braids to one, or something like that, can't remember exactly.